Pharmacological Mechanism Of Long-chain Peptide Class Of Scorpion Neurotoxin

In the present study, the modulation of BmK I and BmK AS, the two putativelong-chain scorpion toxins purified from Buthus martensi Karsch (BmK) venom, onvoltage-gated sodium channels was investigated using whole-cell recording, fluorescenceimaging and behavior experiments to reveal the potential selectivity for sodium channelsubtypes and the underlying mechanism of toxins-induced nociception/anti-nociception.The Selective Modulation of BmK I on Sodium Channel SubtypesWhole-cell patch-clamping was used to record the tetrodotoxin-sensitive (TTX-S)and tetrodotoxin-resistant (TTX-R) components of voltage-gated Na+ currents in smallDRG neurons. It was found that the inhibitory effect of BmK I on open-state inactivationof TTX-S Na+ currents was stronger than that of TTX-R Na+ currents. In addition, BmKI exhibited a selective enhancing effect on voltage-dependent activation of TTX-Scurrents, and an opposite effect on time-dependent activation of TTX-S and TTX-R Na+currents. The results suggested that the inhibitory effect of BmK I on open-stateinactivation might contribute to the increase of peak TTX-S and TTX-R currents, and theenhancing effect of BmK I on time-dependent activation might also contribute to theincrease of peak TTX-S currents. Furthermore, BmK I strongly prolonged theinactivation of INa expressed on transiently-NGF-induced PC12 cells and largelyincreased its peak INa. The cytosolic Na+ fluorescence imaging displayed BmK I lackedeffect on the resting channel but remarkably enhanced the veratridine modification onTTX-sensitive sodium channel. The data suggested BmK I preferentially affectTTX-sensitive sodium channel and facilitate the allosteric modulation of site 2 toxin onTTX-sensitive sodium channel.The Anti-nociception of BmK AS and the Possible Mechanism on SpecificallyModulating Voltage-Gated Na+ Currents in Primary Afferent NeuronsBmK AS produced significant antinoceptive effects on spontaneous pain inducedby formalin when subcutaneously injected into the rat hindpaw 1 min before or 10 minafter formalin injection. Using the whole-cell patch-clamp recording, exposure of acutelyisolated sensory neurons to BmK AS produced one-fold decrease in the number of actionpotentials (APs) evoked by a ramp of depolarizing current. To investigate the mechanismof action of BmK AS, isolated membrane current and Ca2+ influx were examined. BmKAS produced insignificant effect on voltage-dependent IK and KCl or caffeine inducedCa2+ influx, but caused remarkable suppressive effect on tetrodotoxin-resistant (TTX-R)and tetrodotoxin-sensitive (TTX-S) INa. The further experiments showed BmK ASreduced the peak TTX-R and TTX-S Na+ conductance in a dose-dependent manner,prompted the voltage-dependent activation and caused a negative shift of the steady-stateinactivation of TTX-R and TTX-S INa. Thus, the present results indicated theanti-nociceptive response of BmK AS may be ascribed to its specific modulation onvoltage-gated Na+ channels of sensory neurons.